Reduce amount of unwraps

Created by: hcpl

Things have done to achieve that:

• Introducing an error_chain-ish custom error type, convertable from all other ones used in src/event.rs and src/cursor.rs.
• Replacing .unwrap()s in src/event.rs and src/cursor.rs by explicit error return values.
• Modifying public interfaces which is mentioned below.
• Refactoring and fixing formatting.

Changes that can be considered as regressions:

• Using custom error type instead of std::io::Error may add memory and likely but little CPU overhead, though compiler could possibly pull some null-pointer optimizations to completely remove the said overhead. This is also useful for end-users that want to filter the specific error which is hard with io::Error::new(/* whatever io::ErrorKind variant here */, "custom error message").
• Breaking changes in the public cursor::DetectCursorPos trait: cursor_pos method now returns error::Result<(u16, u16)> instead of io::Result<(u16, u16)>.

Possible improvement ideas, their pros and cons:

• Switch completely to error_chain - this introduces the backtrace dependency which may not be suitable for platforms that don't support it, and as a result, makes the error::Error type consume more memory - which was the primary reason to not to use that crate from the beginning. On the other hand, improved maintainability and readability.
• If the breaking change above is accepted, use error::Error and error::Result throughout the crate, not just in src/event.rs and src/cursor.rs, for better interoperability and consistency (if make a breaking change anyway, do it correctly). If not, then convert everything back to io::Error and io::Result which is fine, but in this case custom errors do need heap allocation. And of course, no plans to go back to .unwrap()s (the fewer unexpected panics, the better).