Update regex

Hopefully improve regex of IPython extension
Add multiplication and division to dB

- add multiplication and division to dB unit (only if no physical
dimension is given)
- add dB() convenience function
Fix typos
Update unit definitions

- rename 'min' to 'minutes' as it can conflict with numpy min
- remove double definition of angle units
Merge pull request #1 from juhasch/mergedB

Several updates