fix #28, issues with print in version 2.7

[sauln]

This implements a fix for issue #28. It seems to be caused becauseprint is a statement in 2.7 but a function in 3. Importing print function fixed the issue.

I've tested the digits example on 2.7 and 3.6 and everything seems to work fine.

[pep8speaks]

Hello @sauln! Thanks for submitting the PR.

• In the file umap/umap_.py, following are the PEP8 issues :

Line 191:80: E501 line too long (80 > 79 characters)
Line 291:21: E122 continuation line missing indentation or outdented
Line 292:21: E122 continuation line missing indentation or outdented
Line 293:21: E122 continuation line missing indentation or outdented
Line 294:21: E122 continuation line missing indentation or outdented
Line 295:21: E122 continuation line missing indentation or outdented
Line 305:80: E501 line too long (80 > 79 characters)
Line 344:1: E303 too many blank lines (3)
Line 375:80: E501 line too long (80 > 79 characters)
Line 402:1: E303 too many blank lines (3)
Line 445:80: E501 line too long (80 > 79 characters)
Line 655:16: E225 missing whitespace around operator
Line 668:52: E128 continuation line under-indented for visual indent
Line 850:5: E741 ambiguous variable name 'I'
Line 868:80: E501 line too long (80 > 79 characters)
Line 1029:55: E203 whitespace before ':'
Line 1112:80: E501 line too long (80 > 79 characters)
Line 1113:9: E722 do not use bare except'
Line 1234:80: E501 line too long (80 > 79 characters)
Line 1260:80: E501 line too long (80 > 79 characters)