Register the electron unit with units framework

Closes #105 and allows initialization like unit="electron"
Update docs to reflect registration of electron unit
Trim mask and uncertainty along with image

Closes #100
Trim uncertainty in trim_image by fits_slice

Closes #110
Describe lack of bounds checking in numpy indexing

Closes #53
Merge pull request #111 from mwcraig/trimming-fixes

Trimming fixes
Merge pull request #109 from mwcraig/electron-unit-patch

Electron unit patch

Merging even though it will require an update once astropy 0.4 is out.
Use unit from FITS header of image
Document reading unit from FITS file

Closes #51
Merge pull request #113 from mwcraig/read-image-unit

Read image unit from header if ``BUNIT`` keyword is present
Allow uncertainty to be set with an array

The array is converted to a SdtDevUncertainty. Closes #81
Merge pull request #114 from mwcraig/uncertainty-setter

Allow uncertainty to be set with an array
Use multiply method to scale dark

Closes #108
Merge pull request #115 from mwcraig/sub-dark-better

Use multiply method to scale dark