Escape reserved characters teamcity reporter messages

Teamcity fails to report tests which contain an unescaped reserved
character in the message text. Now the teamcity reporter escapes each of
these characters
first draft for with_mock function
fix build warning: \em doesn't seem to be valid Rd
Merge branch 'trivial-doc' into 159-mocking
attempt to fool R CMD check
remove ligature
Merge branch 'trivial-doc' into 159-mocking
supply example
Merge pull request #161 from krlmlr/trivial-doc

fix build warnings concerning package documentation
use add = TRUE for on.exit
correctly implement checking that all arguments are named
test for mocking non-existing function
support qualified mock names of the form `<package>::<function>`

- mock only functions, even if no package is specified
- .env argument remains in place
- supply tests
rewrite to use one single list of objects of class 'mock'
avoid variable name collision
always test temporary nature of mock
use qualified notation in example
use union merge driver for NEWS.md
Merge branch 'union-merge' into 159-mocking
add NEWS item