This fixes many problems related to extension management:

- introduces extension conflict resolution
- introduces new way to manage extension state (disabled/enabled)
- introduces better extension.so handling solves #154

TODO:

- automated tests
- could not overwrite `phpbrew ext disable <extension>` command
fixed new disable command
moved ExtensionInterface to it's own file
PSR-1 PSR-2 code standard fixes
cleanning (dead) methods from PhpBrew\Utils class
code consistency fix, no snake_case variables on class properties and method declarations
updated completion for `phpbrew ext` subcommands, so only pertinent extensions are suggested.
compatibility with php 5.3
Merge pull request #156 from marcioAlmada/develop

Fixing problems related to extension management.