Implemented TryGetSection + minor changes.

Added `TryGetSection` methods to `Elf` and `IELF` classes. Reimplemented
associated `Get` methods to use TryGets.

Defined magic index with a meaninigful name private constant.

Renamed template parameter in `GetSections` from `S` to `TSections`.
Merge branch 'Implement_TryGetSection_in_ELF' of https://github.com/luk32/elfsharp
Fixes for PR #23.
Updated version.