Add custom loader for Powershell, improve its implementation in Cmder

Let me know what do you think about this! :smile:

Fixes #104, fixes #53, fixes #65 (my bad), related to #136, and should
fix #139

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![capture](https://cloud.githubusercontent.com/assets/846943/4300150/09b8c9b4-3e41-11e4-8c46-53914675a939.PNG)

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Some notes:
* If you get warning at the first execution open PS as admin and run `Set-ExecutionPolicy RemoteSigned`
* To get git functionality, run `Install-Module posh-git`
* PsGet is include for module manager
* Alias are not yet implemented (I'm not sure how to)
* My availbility can be really inprevisible, so please consider cloning it if you want to alter :baggage_claim: