This commit refactors the block checksumming by removing the
iteration over each block and instead only checking blocks which
have data. This requires merging the cache inspection with the
roaring iterator to reduce CPU time and memory allocations.
This commit updates the import path from:
```
pilosa/*
```
to:
```
github.com/umbel/pilosa/*
```
This follows standard Go convention and makes the project "go gettable".
The README was also updated to reflect the changes.
