Fix documentation for %--%
A complete rewrite of the lubridate parser.

Base the parser on gsub and strptime instead of str_split, strsplit etc. It's
considerably faster and more consistent.

Replace the workhorse parse_date with parseDateTime.

Support for more formats.

Heterogeneous date-time formats in a single vector are handled correctly and not
just ignored.
remove unnecessary sub-setting in new_period, period
adjust test cases for new error message and new formats
first take on  documentation
... in parser functions can contain character as well as numeric vectors
Doc tweaking.
correctly handle train/no-train error messages + for efficiency reasons skip
training in ymd and xxx_hms family of functions if length(x) < 300.
added deprecation for internal function parse_date
fixed bug in parseDateTime for train = NULL
many more examples and improved docs
separated formats have priority over collapsed + improved handling of separators
allow fractional seconds in Period class
a smarter training set selection + docs
better treatment of truncated formats

truncated formats are appended in order given by sorted trained formats and not
at the end of formats' list
test cases to accommodate new features
documented lubridate_formats + other doc fixes
re-roxygenise
add commented speed test to test-parsers.R
better order of lubridate_formats