invalid Chemspider html syntax

[stanstrup]

Evidently CS is sometimes not escaping properly special characters.
Here is a fix for the problem I found. I could not find a general way to
fix this. I guess if there was one parsers would do it.

Example here: https://www.chemspider.com/Chemical-Structure.8012.html

The line: "MP (exp database): <-80 deg C" makes the parser terminate the tag so the rest is lost and things are not as cs_prop expects.

I added to the PR here: #136
I hope that is OK.

[eduardszoecs]

I cherry-picked (and droped the dependency on curl).