added optional psr4 prefix to configuration

Added optional ps4-prefix to the PSR0 Locator.

It is now possible to specify the prefix in your config like so:
```
suites:
  behat_suite:
    namespace: Behat\Tests\MyNamespace
    psr4_prefix: Behat\Tests
```

When creating and/or manipulating source files, phpspec will cut the prefix. It will be retained in the spec folder structure.

This is for example needed, if you want to develop for a composer library, that uses the new PSR4 autoloading mechanism and therefore hasn't all the namespace parts represented in its folder structure.