LOG4PHP-153: Added bufferSize parameter to LoggerAppenderMail

This parameter determines how many messages are buffered before the buffer
is sent to the recipient. If bufferSize is not set, the appender will send
all messages together on close (same as prior behaviour, so it remains
backward compatible).

Based on patch by Michal Vanek (see JIRA issue).

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
LOG4PHP-153: Updated LoggerAppenderMail docs

Added docs for bufferSize parameter and buffering.

Removed bit which recommended using LoggerAppenderMailEvent which will
soon be made deprecated.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
LOG4PHP-153: Made LoggerAppenderMailEvent deprecated

A warning will be issued if used. Added a warning to docs.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>