Modified default range to interpret the assumptions of free_vars

Currently plot has a default range of (-10,10) when user doesn't provide a range. However this default behaviour doesn't care about the properties/assumptions of the free_variables. This commit corrects
this situation for two such cases, ie, when the variable is positive and when it is negative.

I'm still working to figure out a general method to do it. Perhaps a function that calculates the range over which a variable is defined(taking into account its assumptions) will do. I couldn't find such a function in the assumptions module. @skirpichev  
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