BUG: Warn when dtypes differ in between chunks in csv parser

closes #3866

Silently fix problem rather than warning if we can coerce
to numerical type.
Merge branch 'issue-3866-column-conversion' of https://github.com/guyrt/pandas into guyrt-issue-3866-column-conversion

Conflicts:
	doc/source/release.rst
	pandas/io/tests/test_parsers.py