added the function extract_waveunit to find out which unit was used in a FITS header to measure wave lengths
typo
save the wavelength unit in the header in the key WAVEUNIT if it could be read
set the WAVEUNIT value actually in the fits module, not in file_tools
Merge pull request #536 from sunpy/0.3

Merge 0.3 into Staging
Merge branch 'staging' into extract_waveunit
Merge branch 'staging' of https://github.com/sunpy/sunpy into extract_waveunit
support more formats for detecting the waveunit
detect_waveunit: support all exponents from -1 to -24
Merge branch '0.3' of https://github.com/sunpy/sunpy into extract_waveunit

Conflicts:
	sunpy/io/fits.py
added example in documentation of extract_waveunit
extend fits.get_header documentation
say that the key WAVEUNIT is written and contains the wavelength unit which is used
to measure the wavelength (used in the value of WAVELNTH).
fixed example of extract_waveunit
Merge pull request #522 from derdon/extract_waveunit

new function extract_waveunit in module io.fits