Add a method to do HTTP POST

This was contributed by Steven Kroon
Send the bytes in doHTTPPOST2 in a simple for-loop

Also we set the doHTTPPOST return value to true, for the time being. We'll
have to check the StatusCode in +HTTPACTION:1,<StatusCode>,<DataLen>
Rename the common ("1" and "3") HTTP functions to prolog and epilog
Increase timeout for HTTPACTION to 20s
Rename the "middle" part HTTP functions
Rename the sendCommand to make it more clear what they do
Create separate function for HTTPREAD and HTTPACTION

This functions can be used for GET and POST
More uses of doHTTPACTION
Add doHTTPPOSTmiddleWithReply which also reads the data from the server

This function does the same as doHTTPPOSTmiddle, but it also reads the
data that was sent by the server.
Move some functions around
Add function doHTTPPOSTWithReply