Make curl optional for installing yen

[ChillarAnand]

Many bootstrapping scripts provide instructions for installing the tool via curl or get.

Currently, curl is mandatory for installing yen. It would be great if it can make use of wget when it is present.

[tusharsadhwani]

curl is used inside the install script as well, so I'll have to edit the script to fallback to wget when curl isn't available.

Would you like to try doing this change?