[Downloader::Git] Cache repos

Closes #105, #121, #215.

Still need to do it at the target level too.

* master:
  [Command::Spec::Linter] fix for #podspec_warnings.
  [Command::Spec] Template layout fix.
  [Platform] Removed options accessor.
  [Executable] Made executed command more prominent.
  Spelling correctioSpelling correctionn
  [Resolver] Support for deployment target check.
  [Specification#deployment_target=] Switch to Pod::Version
  [Platform#==] Check for deployment target.
  [Platform] Simplify deployment target specification.
  [#212] Check for deployment target

[#212] Check for deployment target

- ensure that cache git repos are clean
- added cache size limit

- Allow to specify the deployment target after initialization

* master:
  [Installer] Don't generate documenation if already installed
  [Command::Spec::Linter] Fix - Skip documentation generation
  [Config#doc] Renamed to generate_docs
  [Command::Spec::Linter] Bug fix.
  [# 242 Command::Spec::Linter] Refactoring
  [#246 | Resolver] Validate plaforms against each target.
  [Resolver] Check target plafroms instead of podfile.

Conflicts:
	lib/cocoapods/resolver.rb

* master:
  [Installer] Don't generate documenation if already installed

It checks that the documenation exist for a given version of a pod
and in that case it doesn't generate it again.

* master:
  [Command::Spec::Linter] Fix - Skip documentation generation
  [Config#doc] Renamed to generate_docs
  [Command::Spec::Linter] Bug fix.

- moved Specification#validate! to Linter
- fixed FileList bugs
- cleaned up