import menpo is now twice as fast

Based on a [few experiments I ran](http://nbviewer.ipython.org/gist/jabooth/cf6d0803422068f2d3f2) this PR inlines a few expensive imports to speed up `import menpo` by around a factor of 2. There is no change to the API and usage of Menpo - we are simply being a little more lazy in loading code.

Master:
```python
%%time
import menpo
CPU times: user 320 ms, sys: 64.2 ms, total: 384 ms
Wall time: 401 ms
```
This PR:
```python
%%time
import menpo
CPU times: user 101 ms, sys: 39.4 ms, total: 141 ms
Wall time: 157 ms
```
Does this mean we pay a penalty here everytime we use `Image.gaussian_pyramid`?